A powershift transmission is understood to be a variable-speed transmission in which a gearshift takes place under load, i.e. with no interruption of the force transfer between the driveshaft of the drive engine and the output shaft of the transmission. During this, the force flow in a gearshift is transmitted by the actuation of frictional shifting elements from a currently load-bearing force transmission branch with a current, actual gear ratio, by operating with slip for a short time, continuously to a parallel force transmission branch with a target gear ratio. Known structures for powershift transmissions are the planetary automatic transmission and the dual clutch transmission.
In a planetary automatic transmission the shift-related force transmission transfer takes place by the actuation of transmission-internal frictional shifting elements such as shifting clutches and shifting brakes, in that with time overlap, at least one friction shifting element that is load-carrying when the loaded gear is engaged, is opened, and at least one friction shifting element that carries the load when the target gear is engaged, is closed.
In a dual clutch transmission the shift-related force transmission transfer takes place by actuating the transmission-external friction clutches arranged between the driveshaft of the drive engine and, respectively, one of the two input shafts, in that first the target gear is engaged in the partial transmission that is not carrying the load and then, with time overlap, the friction clutch associated with the gear under load is opened and the friction clutch associated with the input shaft of the target gear is closed. The typical structure of a dual clutch transmission and a method for controlling shifts in a dual clutch transmission, in which a traction upshift takes place without any electronic action in the engine control system, can be seen in DE 10 2004 005 789 A1.
The disadvantages of shifting processes in the known powershift transmissions are, for one, the need to operate with frictional slip during the phase of force transfer from the force transmitting branch containing the initially loaded gear to the force transmitting branch containing the target gear. The frictional slip in the friction shifting elements or friction clutches dissipates drive energy, i.e. converts it into heat. Likewise, the frictional slip necessarily results in wear of the friction linings of the friction shifting elements and clutches concerned. The dissipation of drive energy increases the fuel consumption of the motor vehicle concerned, and wear on the friction lining limits the useful life of the powershift transmission.
To avoid these disadvantages, in DE 10 2006 036 758 A1 a dual clutch transmission is proposed, in which the input shafts of the two partial transmissions can in each case be connected in a rotationally fixed manner to the driveshaft of the drive engine in each case by means of a main separator clutch in the form of an unsynchronized claw clutch. In that case, as starting and synchronization means two electric machines are provided, each of them in driving connection with one of the two input shafts. Alternatively a single electric machine can also be provided, which can be brought into driving connection with one or other of the two input shafts in alternation by means of a shifting mechanism. It is true that this design of a dual clutch transmission does avoid slipping operation of the otherwise common friction clutches. However, the disadvantage of a dual clutch transmission of such a design is the high outfitting cost of the two electric machines or of the one electric machine together with the shifting mechanism, as well as at least one electrical energy accumulator and the associated control technology.